Cosmic Data: Setting a Mass Ceiling for Dark Matter Black Holes

Physicists at the Mainz Institute for Theoretical Physics and New York University Abu Dhabi have determined that black holes heavier than 100 billion times the mass of the sun cannot make up the majority of dark matter. The study uses light from the early universe to place a strict upper limit on the size of these invisible objects, effectively ruling out the largest candidates for the mysterious substance that holds galaxies together.

Dark matter is estimated to constitute about five-sixths of all matter in the cosmos. It was proposed decades ago to explain why galaxies rotate so fast without flying apart. While the substance is invisible, its gravitational pull is observable. Researchers had long considered black holes as a potential source, given their mass and difficulty in detection. However, no object larger than about 36 billion solar masses has been found yet.

The team focused on what they call stupendously large black holes, objects with more than 100 billion solar masses. Some theoretical models suggest these could weigh between 1 million billion and 10 million billion solar masses, enough to outweigh entire galaxies. Christopher Gerlach, a theoretical physicist at the Mainz Institute, noted that dark matter is often pictured as a fine dust spread everywhere. If it were instead clumped into a few gigantic objects, he said, the universe would become grainy on very large scales.

To test for this graininess, the researchers analyzed the cosmic microwave background. This is the light released when the universe was roughly 380,000 years old. At that time, the cosmos was filled with hot plasma that trapped light. As the universe expanded and cooled, atoms formed and light was free to travel. Gerlach described this light as a snapshot of the infant universe covering the whole sky.

Antonio Iovino, a theoretical physicist at New York University Abu Dhabi, explained that the temperature of this background varies from place to place by about one part in 100,000. These tiny fluctuations map how matter was distributed in the early universe. If massive black holes had been abundant, they would have created a distinct pattern in this data.

Nicholas Leister, a theoretical physicist at the Mainz Institute, stated that the background lacks the expected graininess. The analysis showed that black holes over the 100 billion solar mass mark cannot be a significant part of the dark matter. The authors clarified that this does not mean such holes cannot exist, only that they are not the primary component. They are now investigating whether smaller black holes, about the size of asteroids, might still be a major constituent.

The findings are scheduled for publication on October 26 in the journal Physical Review Letters.